The use of liquid carbon dioxide for producing CO.sub.2 snow and subsequently accelerating it to high speeds for cleaning minute particles from a substrate is taught by Layden in U.S. Pat. No. 4,962,891. A saturated CO.sub.2 liquid having an entropy below 135 BTU per pound is passed though a nozzle for creating, through adiabatic expansion, a mix of gas and the CO.sub.2 snow. A series of chambers and plates are used to improve the formation and control of larger droplets of liquid CO.sub.2 that are then converted through adiabatic expansion to the CO.sub.2 snow. The walls of the ejection nozzle for the CO.sub.2 snow are suitably tapered at an angle of divergence of about 4 to 8 degrees, but this angle is always held below 15 degrees so that the intensity of the stream of the solid/gas CO.sub.2 will not be reduced below that which is necessary to clean the workpiece. The nozzle may be manufactured of fused silica, quartz or some other similar material.
However, this apparatus and process, like other prior art technologies, utilizes a Bernoulli process that involves incompressible gasses or liquids that are forced through a nozzle to expand and change state to snow or to solid pellets. Also, the output nozzle functions as a diffusion promoting device that actually reduces the exit flow rate by forming eddy currents near the nozzle walls. This mechanism reduces the energy and the uniformity of the snow distributed within the exit fluid, which normally includes liquids and gasses as well as the solid snow.
Some references, such as Lloyd in U.S. Pat. No. 5,018,667 at columns 5 and 7, even teach the use of multiple nozzles and tapered orifices in order to increase the turbulence in the flow of the CO.sub.2 and snow mixture. These references seek to disperse the snow rather than to focus it after exiting the exhaust nozzle. At column 7, lines 34-51, Lloyd indicates that the snow should be created at about one-half of the way through the nozzle in order to prevent a clogging or "snowing" of the nozzle. While Lloyd recognizes that the pressure drop in a particular orifice is a function of the inlet pressure, the outlet pressure, the orifice diameter and the orifice length, his major concern was defining the optimum aspect ratio, or the ratio of the length of an orifice to the diameter of the orifice, in order to prevent the "snowing" of the orifice.
A common infirmity in all of these references is that additional energy must be provided to accelerate the snow to the desired exit speed from the nozzle when the snow is not created in the area of the exhaust nozzle.
Therefore, it is a primary object of the present invention to create the CO.sub.2 snow at a location downstream of the throat in the nozzle such that the supersonic speed of the CO.sub.2 will be transferred to the snow, while simultaneously focusing the snow and the exhaust gas into a fine stream that can be used for fineline cleaning applications.